Remove the meat from the refrigerator for 1-2 hours prior to cooking.,Preheat the oven to 500 F and prepare kitchen string (a long enough piece of string to wrap around the roast several times) and a roasting pan with a rack (if you have a rack, use it; if not, dont fret).,In a large shallow dish, combine the salt, pepper, garlic powder and onion powder.,______,Place the prime rib in the roasting pan (fat side up) on the rack and tie around it a few times with kitchen string.,Roast the prime rib fat-side up at 500 F for 15 minutes, then reduce the heat to 250 F to keep cooking.,Cook for 45 minutes to 1.5 hours more until the internal temperature of the meat reaches 125 F for rare, 135 F for medium-rare, and 145 F for medium.,When done, remove the roasting pan from the oven and set it on a rack.,Let the roast rest for 30 minutes before removing the string, slicing and serving.,For the horseradish cream, combine all ingredients in a bowl using a whisk or in a small food processor.,Taste, and add a pinch of salt if needed and more horseradish to your taste.,Serve with the prime rib.,Notes: The spice mixture may be added to the beef several hours before or the night before for extra flavor.,To make ahead, roast it to 10 or so degrees below the desired cooked temperature.,To reheat, place it uncovered on a roasting rack in a roasting pan in a 350 F oven and cook until the outside is crispy and sizzling.